std::transform over QMap

[Venkateswaran]

I have to transform a QMap<QString, int>. I tried to use std::transform as it works for QList, but it's failed to compile.

#include <QCoreApplication>
    #include <algorithm>
    #include <QMap>
    
    int main(int argc, char *argv[]) {
      QCoreApplication a(argc, argv);
    
      QMap<QString, int> map;
      map["one"] = 1;
      map["three"] = 3;
      map["seven"] = 7;
    
      QMap<QString, int> map2;
    
      std::transform(map.begin(), map.end(), std::back_inserter(map2), [](const QPair<QString, int>& item) {
          return qMakePair(item.first+"testing", item.second);
      });
    
      return a.exec();
    }

I'm getting the below error while compiling the above code. I would like to achieve this without general for loop as my project using cppcheck and it throws warnings.

   /usr/include/c++/7/bits/stl_iterator.h:490: error: no type named ‘value_type’ in ‘class QMap<QString, int>’
    In file included from /usr/include/c++/7/bits/stl_algobase.h:67:0,
                     from /usr/include/c++/7/algorithm:61,
                     from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qglobal.h:142,
                     from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qcoreapplication.h:43,
                     from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/QCoreApplication:1,
                     from ../test/main.cpp:1:
    /usr/include/c++/7/bits/stl_iterator.h: In instantiation of ‘class std::back_insert_iterator<QMap<QString, int> >’:
    ../test/main.cpp:15:65:   required from here
    /usr/include/c++/7/bits/stl_iterator.h:490:7: error: no type named ‘value_type’ in ‘class QMap<QString, int>’
           operator=(const typename _Container::value_type& __value)
           ^~~~~~~~
    
    /usr/include/c++/7/bits/stl_algo.h:4306: error: no match for call to ‘(main(int, char**)::<lambda(const QPair<QString, int>&)>) (int&)’
    In file included from /usr/include/c++/7/algorithm:62:0,
                     from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qglobal.h:142,
                     from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/qcoreapplication.h:43,
                     from ../../../../Qt5.12.4/5.12.4/gcc_64/include/QtCore/QCoreApplication:1,
                     from ../test/main.cpp:1:
    /usr/include/c++/7/bits/stl_algo.h: In instantiation of ‘_OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation) [with _IIter = QMap<QString, int>::iterator; _OIter = std::back_insert_iterator<QMap<QString, int> >; _UnaryOperation = main(int, char**)::<lambda(const QPair<QString, int>&)>]’:
    ../test/main.cpp:17:4:   required from here
    /usr/include/c++/7/bits/stl_algo.h:4306:24: error: no match for call to ‘(main(int, char**)::<lambda(const QPair<QString, int>&)>) (int&)’
      *__result = __unary_op(*__first);
                  ~~~~~~~~~~^~~~~~~~~~
    /usr/include/c++/7/bits/stl_algo.h:4306:24: note: candidate: QPair<QString, int> (*)(const QPair<QString, int>&) <conversion>
    /usr/include/c++/7/bits/stl_algo.h:4306:24: note:   candidate expects 2 arguments, 2 provided
    ../test/main.cpp:15:102: note: candidate: main(int, char**)::<lambda(const QPair<QString, int>&)>
       std::transform(map.begin(), map.end(), std::back_inserter(map2), [](const QPair<QString, int>& item) {
                                                                                                          ^
    ../test/main.cpp:15:102: note:   no known conversion for argument 1 from ‘int’ to ‘const QPair<QString, int>&’

[Christian Ehrlicher]

@Venkateswaran said in std::transform over QMap:

std::back_inserter(map2)

How should this work? QMap has no push_back() and can't have it since it needs a key and a value.

[Venkateswaran]

@Christian-Ehrlicher is there any other way to achieve this without general for loop?

[Christian Ehrlicher]

No, but why?

for (auto it = map.begin(); it != map.end(); ++i)
  map2.insert(it.key + "testing", it.value);

[Venkateswaran]

@Christian-Ehrlicher Because of cppcheck in my project :) it's not allowing me to use for loop for this.

[jsulm]

@Venkateswaran said in std::transform over QMap:

it's not allowing me to use for loop for this

?
Can you post what it tells you?

[Christian Ehrlicher]

@Venkateswaran said in std::transform over QMap:

it's not allowing me to use for loop for this.

It's just suggesting to use std functions when possible. But it is not. Blame cppcheck.

[qwertzui11]

I guess the correct code could would look like

void fun() {
  QMap<QString, int> map;
  QMap<QString, int> map2;
  std::transform(map.begin(), map.end(), std::inserter(map2, map2.end()),
                 [](auto &todo) { return todo; });
}

however there seems to be no QMap<QString, int>::value_type defined. Qts container seem do not stick to the standard, so the std::inserter paradigma does not work.

using the standard map, it compiles.

void works() {
  std::map<QString, int> map;
  std::map<QString, int> map2;
  std::transform(map.begin(), map.end(), std::inserter(map2, map2.end()),
                 [](auto &todo) { return todo; });
}

Have fun
Markus

[Christian Ehrlicher]

@qwertzui11 said in std::transform over QMap:

Qts container seem do not stick to the standard,

correct because QMap was there earlier :)

A range-based for-loop for QMap similar to std::map can be done with https://doc.qt.io/qt-5/qmap-key-iterator.html afair

[Venkateswaran]

Ok. My actual conversion is from QMap<quint16, QPair<QString, quint16>> to QMap<quint16, QPair<QUuid, quint16>> and QHash<QString, int> to QHash<QUuid, int>

QMap<quint16, QPair<QUuid, quint16>> map2;
for(auto it = map.begin(); it != map.end(); ++it)
    map2.insert(it.key(), qMakePair(QUuid(it.value().first), it.value().second));

QHash<QUuid, int>map2;
for(auto it = map.begin(); it != map.end(); ++it)
    map2.insert(QUuid(it.key()), it.value());

and the cppCheck not complaining. Looks like cppcheck complain only if didn't do much inside the loop (I'm not sure).

I get the below warning when I convert QList<enum> to QList<QString>.

Controller.cpp:120:21: warning: Consider using std::transform algorithm instead of a raw loop. [useStlAlgorithm]
      versionsInStr.push_back(convertApiVersionToString(ver));

Thank you @Christian-Ehrlicher

[qwertzui11]

@Christian-Ehrlicher agreed. I hope Qt6 fixes these smalls gaps :-)

[Christian Ehrlicher]

@qwertzui11 It can't since it will break a lot of existing stuff - you won't explain all customers that the range-based for loop for containers now returns an iterator instead the value on a QMap and that they have to adjust all of it's code. :(

[SGaist]

Hi,

You can use QMap:: keyValueBegin and QMap::keyValueEnd for your use case.